The present invention relates to a steering apparatus for a vehicle which is capable of varying the ratio between amount of operation of an operating member and amount of steering of vehicle wheels.
A vehicle steering apparatus has been developed which is capable of varying a steering characteristic by varying the ratio between the amount of operation of the operating member and the amount of steering of the vehicle wheels, when movement of a steering actuator is caused by operation of the operating member and transmitted to the vehicle wheels in order to vary the steering angle. An apparatus employing a so-called steer-by-wire system in which the operating member is not mechanically connected to the vehicle wheels and an apparatus in which the operating member is mechanically connected to the wheels are known as such a steering apparatus. In a steering apparatus employing the steer-by-wire system, without mechanically connecting the operating member in the form of a steering wheel to the vehicle wheels, the ratio between the amount of operation and the amount of steering is varied by controlling the steering actuator, when movement of the steering actuator is transmitted to the vehicle wheels in order to vary the steering angle through a steering gear. In the steering apparatus in which the operating member is mechanically connected to the wheels, the ratio between the amount of operation and the amount of steering is varied, when rotation of the steering wheel is transmitted to the steering gear through a variable transmission ratio mechanism such as a planetary gear mechanism by controlling the steering actuator which drives a ring gear or the like comprising the planetary gear mechanism.
In the steering apparatus employing the steer-by-wire system, external forces based on friction with the road surface such as steering resistance and self-aligning torque are not transmitted to the operating member. In the steering apparatus in which the steering wheel and the vehicle wheels are mechanically connected via the variable transmission ratio mechanism, such steering resistance and self-aligning torque do not correspond to the amount of operation of the operating member. Hence by providing an operating actuator for generating torque which acts upon the operating member, a driver is provided with steering feeling.
It has been proposed that when torque is caused to act upon the operating member by the operating actuator in the vehicle steering apparatus which is capable of varying the ratio between the amount of operation and the amount of steering, this torque is increased and decreased in accordance with the amount of operation. This is to improve driving stability at high speeds and maneuverability at low speeds. In this case, due to the delay that exists between the operation of the operating member and the actual variation in the steering angle of the vehicle, the driver cannot be provided with steering feeling which is matched to the actual variation in steering angle, only by increasing or decreasing the torque which acts upon the operating member in accordance with the amount of operation. This delay between the operation and the actual variation in steering angle is caused by external forces such as steering resistance and self-aligning torque which act on the vehicle wheels on the basis of friction with the road surface. Hence, a method of controlling the operating actuator in consideration of the effects of such external forces has been under consideration to compensate for the delay between the operation and the variation in steering angle and to provide the driver with a steering feeling which is matched to the actual variation in steering angle.
Conventionally, when considering the effects of such external forces, a control system for the operating actuator is constructed based on supposed inertia, viscosity, elasticity, friction and others in the steering force transmission system in a conventional steering apparatus in which the steering wheel and the vehicle wheels are mechanically connected such that the ratio between the amount of operation and the amount of steering cannot be varied. However, characteristics of such inertia, viscosity, elasticity, friction and others differ from vehicle type to vehicle type and from steering apparatus to steering apparatus, causing a problem in that the time and resources required for developing the control system for operating actuator increase.
An object of the present invention is to provide a vehicle steering apparatus which is capable of solving the above problem.
A vehicle steering apparatus of the present invention comprises: an operating member; an operating actuator for generating torque which acts upon the operating member; a steering actuator; a steering gear for transmitting movement of the steering actuator to vehicle wheels so that variation in steering angle is generated; a sensor for detecting amount of operation of the operating member; a sensor for detecting amount of steering of the vehicle wheels; means for controlling the steering actuator such that the ratio between the amount of operation of the operating member and the amount of steering of the vehicle wheels is varied; means for determining behavior index value for operating corresponding to a change in behavior of the vehicle caused by change in steering angle; means for storing a relationship between the amount of operation of the operating member and target behavior index value for operating; means for calculating the target behavior index value for operating from the stored relationship and the detected amount of operation; means for storing a relationship between the behavior index value for operating, the target behavior index value for operating, and target operating torque; means for calculating the target operating torque from the stored relationship, the determined behavior index value for operating, and the determined target behavior index value for operating; and means for controlling the operating actuator such that operating torque of the operating member corresponds to the target operating torque.
According to the present invention, torque is caused to act upon the operating member by controlling the operating actuator on the basis of change in behavior of the vehicle caused by a change in steering angle, and thus a driver can be provided with steering feeling which is matched to the actual change in steering angle. Furthermore, since there is no need to consider inertia, viscosity, elasticity, friction and others in the steering force transmission system, time and resources required for developing a control system for the operating actuator can be greatly reduced.
It is preferable that the target operating torque includes a component value corresponding to a deviation obtained by subtracting the behavior index value for operating from the target behavior index value for operating. Therefore the operating torque can be increased in a transient state in which the behavior index value for operating has not reached the target behavior index value for operating, and thus divergence in the behavior of the vehicle can be prevented.
It is preferable that the component value corresponding to the deviation obtained by subtracting the behavior index value for operating from the target behavior index value for operating varies in accordance with the vehicle speed. Therefore operating torque can be reduced at low speeds to facilitate steering.
It is preferable that the vehicle steering apparatus of the present invention comprises means for determining behavior index value for steering corresponding to change in behavior of the vehicle caused by change in steering angle, means for storing a relationship between the amount of operation of the operating member and target behavior index value for steering, means for calculating the target behavior index value for steering from the stored relationship and the detected amount of operation, means for storing a relationship between the behavior index value for steering, the target behavior index value for steering, and target amount of steering, and means for calculating the target amount of steering from the stored relationship, the determined behavior index value for steering, and the determined target behavior index value for steering, wherein the steering actuator is controlled such that the amount of steering of the vehicle wheels corresponds to the target amount of steering.
Thus the steering actuator can be controlled on the basis of change in behavior of the vehicle caused by change in steering angle, and since there is no need to consider inertia, viscosity, elasticity, friction and others in the steering force transmission system, time and resources required for developing a control system for the steering actuator can also be greatly reduced.
In the vehicle steering apparatus of the present invention, the means for determining the behavior index value for operating and the means for determining the behavior index value for steering preferably comprise a sensor for detecting value which corresponds to acceleration of change in behavior of the vehicle in at least one direction caused by change in steering angle.
In the vehicle steering apparatus according to the present invention, which is capable of varying the ratio between amount of operation of an operating member and amount of steering of vehicle wheels, time and resources required for developing a control system can be greatly reduced.